To maximize the fuel efficiency of motor vehicles equipped with torque converters, it has been increasingly preferred to install a lock-up clutch which substantially rigidly engages the torque converter to positively prevent slippage of the torque converter. With regard to such motor vehicles, it was previously proposed in Japanese patent laid-open publication No. 5-262169 to compute a target output torque of the driven road wheels (axle torque) according to the current vehicle speed and the given throttle opening of the internal combustion engine of the vehicle, and to control the gear ratio of the power transmission system and the throttle opening of the engine so as to achieve the target output torque by taking into account the operating condition of the lock-up clutch.
A lock-up clutch is known to demonstrate partially engaged conditions involving varying degrees of slippage depending on the operating condition of the motor vehicle. However, this previously proposed control system is designed on the assumption that the lock-up clutch is either fully engaged or fully disengaged, and is therefore not capable of performing in an optimum fashion when the vehicle is operating under conditions in which the lock-up clutch is partially engaged in varying degrees.